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When Earth Took a Wallop

In ancient debris, geoscientists detect a stupendous asteroid bombardment.

Kevin Johnson

ROCK OF AGES: Lowe, above, and Sleep found their clues in South Africa and Australia.

AMID VAST TRACTS OF ROCKS in eastern South Africa and Western Australia, there are relatively small areas where methodical exploration and good fortune have paid off with evidence of giant meteorite strikes that took place more than 3 billion years ago.

Donald Lowe, professor of geological and environmental sciences, has been pulling information out of those ancient stones for most of his academic career. But his latest headline, researched in collaboration with geophysics professor Norm Sleep, proclaims one of those disaster-and-discovery stories that make fact more spellbinding than fiction.

Around 3.2 billion years ago, an asteroid at least 12 miles across—and possibly larger than 50 miles—hit the Earth with ocean-boiling, super-earthquake, atmosphere-scorching force. At the minimum size, explains Lowe, the asteroid was twice as big as the one thought to have wiped out the dinosaurs some 65 million years ago. We know all this, he notes, because these particular rocks contain layers of sediment that are composed of distinctive debris produced by such large collisions. "Nobody else has discovered impact layers as old as these," says Lowe, '64. "We're the only ones who have gone out and looked at most of these very ancient beds."

Lowe and several non-Stanford scientists are experts in identifying minute particles that resulted from the condensation and crystallization of rock vapor clouds formed during such powerful impacts. These particles—very different, Lowe points out, from those made of volcanic dust—correlate with very high levels of the element iridium, inferred to come from meteorites (as opposed to the very low levels of iridium found in near-surface terrestrial rocks). A global distribution of these particles can also be inferred and is among the signs of how big these collisions were.

Lowe's fascination with geology stems in part from the ability to look back in time by studying these kinds of rocks. Since 1980, he has been averaging one overseas trip per year to these telltale rock sequences. "I'm no less enthusiastic now than when I started out," he says. And as complex as his and Sleep's calculations are, there remains substantial room for other geologists and geophysicists to build upon and broaden the work.

A better understanding of the size and frequency of such huge impacts during Earth history could further our knowledge of how and when the early continental crust formed and the potential role of large meteorites in that formation. Even more intriguing, perhaps, are questions about the effect of asteroid bombardments on the early evolution of life.

"I have a current student trying to investigate the biological effects of one of these large impacts," Lowe notes. "Giant impacts like these may have driven biological extinctions and revolutions throughout Earth history, right up to and including the extinction of the dinosaurs."

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